It is known to mount hollow protectors on the hub of hypodermic needles as illustrated by U.S. Pat. Nos. 3,390,759; 3,333,682; and 2,677,373. These patents also describe an interfitting rib structure between the protector and needle hub forming a "wrench" effect to rotationally lock the needle to the protector. Thus, a nurse or physician can twistingly wedge the needle hub onto a syringe adapter by twisting the protector without manually touching the protector encased needle.
When the protector is mounted on the needle hub, it must have a proper rotational orientation to the needle hub in order for the interfitting ribs to mesh. This is no problem with manual assembly because the operator can simply twist the protector until the protector ribs fall into place between ribs on the needle hub.
In recent years disposable plastic syringes, which are used for a single injection and then discarded, have come into very wide use. In order to keep the cost of such syringes sufficiently low; i.e., only a few cents, very high speed assembly machines are required. Such machines have a certain degree of vibration that tends to help jostle the syringes and needles into proper alignment. However, sometimes end surfaces on the protector and needle ribs will come into an end to end abutting relationship preventing assembly. When this happens, the assembly machine jams, requiring the efforts of an operator to manually unjam the machine. Such jamming is costly in the manufacture of disposable hypodermic syringes.